An emulsion polymerized PTFE is obtained by polymerizing the tetrafluoroethylene (hereinafter referred to as TFE) monomer, in the presence of pure water, a polymerization initiator, a fluorine-containing anionic surfactant (hereinafter referred to as APFO) having 8 carbon atoms in total such as ammonium perfluorooctanoate and a paraffin wax stabilizer, and it is obtained as an aqueous PTFE polymer emulsion containing PTFE fine particles (Handbook of Fluorine resins, p. 28, edited by Takaomi Satokawa, published by Nikkankogyo Shimbun).
The aqueous PTFE polymer emulsion after the polymerization is used as a low concentration aqueous PTFE dispersion after blended with a nonionic surfactant, as a high concentration aqueous PTFE dispersion after concentration, or as an aqueous PTFE dispersion after blended with a nonionic surfactant, a filler or other known components as the case requires. However, it has not been industrially common to remove APFO from an aqueous dispersion of PTFE.
Because APFO is hardly naturally degradable, it is preferred to minimize the APFO content of products As to means of lowering the APFO content, WO00/35971 (JP-A-2002-532583) proposes a method of preparing an aqueous PTFE dispersion with a low APFO concentration using an ion exchange resin. The resulting aqueous PTFE dispersion having a low APFO concentration has a problem of poor friction stability.
An aqueous PTFE dispersion under friction or shearing tends to get less homogeneous, because the PTFE fine particles fibrillate and form agglomerates. Therefore, an aqueous PTFE dispersion having a low APFO concentration with poor friction stability can clog a feed pump or form an uneven or defective coating containing agglomerates.
In applications which require that an aqueous PTFE dispersion has wettability or has to form thick coatings, it has been conventional to add a large amount of a nonionic surfactant to an aqueous PTFE dispersion. For example, at low APFO concentrations, addition of from 6-12 mass %, based on PTFE, of a nonionic surfactant makes dispersions so viscous that they form problematically thick coatings or lowers the critical cracking thickness (the thickness at which cracking occurs in a thick coating) and has a problem that coatings tend to crack.
WO03/020836 (JP-A-2005-501956) proposes adjustment of the viscosity of an aqueous PTFE dispersion by addition of a fluorine-free anionic surfactant. However, when sodium lauryl sulfate, which is a sulfur-containing anionic surfactant given as an example therein, is used, a coating turns yellowish brown during baking, and sodium sulfate remains as an ionic impurity in the coating after baking. Therefore, it is not suitable for applications to electronic materials such as printed boards.